In medicine, in the case of seriously ill patients so-called oxygenators are used to remove carbon dioxide from the blood of these patients and to oxygenate the blood. Nowadays, oxygenators comprising a membrane by which a blood region is separated from a gas region are used almost exclusively. The blood is taken from a main vessel of the patient and is fed into the blood region of the oxygenator preferably by means of a blood pump. At the same time, a purge gas is transported through the gas region, normally either pure oxygen or a mixture of oxygen and nitrogen being used as a purge gas. In particular, a mixture of 21% oxygen and 79% nitrogen, so-called AIR, is used. The purge gas is in particular taken from wall supplies available in medical facilities and thus meets the requirements of medical gases.
As a result of the pressure gradient of the partial pressure or the concentration gradient of the carbon dioxide, carbon dioxide is transported from the blood region through the membrane and into the gas region, whereas as a result of the partial pressure gradient of the partial pressure of the oxygen or the concentration gradient of the oxygen, the oxygen is transported through the membrane from the gas region into the blood region so that the blood is oxygenated while at the same time carbon dioxide is removed from the blood. The amount of carbon dioxide that is removed from the blood per unit of time, and the amount of oxygen with which the blood is oxygenated per unit of time depend, on the one hand, on the flow rate of the purge gas through the gas region and, on the other hand, on the transport rate of the blood through the blood region.
In known arrangements, the purge gas always comprises oxygen so that an oxygenation of the blood inevitably takes place even if the breathing capacity of the patient himself/herself were sufficient to ensure sufficient oxygen supply, and it were only necessary to remove the carbon dioxide from the blood. The unnecessary enrichment of the blood by means of the oxygenator can result in a hypoxic pulmonary vasoconstriction with a corresponding shunt shift so that the already ill patient is stressed further and his recovery is hindered. Depending on the purge gas used, moreover further gases may be transferred from the purge gas into the blood, which may also result in irritations of the patient.